Project Summary. Through genetic and neurodevelopmental studies, we have discovered that complex strabismus syndromes can be sensitive indicators of human genetic errors in axon growth and guidance. These include horizontal gaze palsy with progressive scoliosis, Duane syndrome, and CFEOM1 that result from mutations in ROBO3, CHN1, and KIF21A, respectively. During the previous grant cycle, we identified the CFEOM3 gene to be TUBB3, which encodes the neuron-specific -tubulin isotype III, and defined this as a fourth disorder of axon guidance. We found that an allelic series of heterozygous missense mutations in this gene cause a spectrum of human nervous system disorders that result from aberrant axon guidance and maintenance. While each mutation causes CFEOM3, some also result in axonal neuropathy, facial paralysis, intellectual and behavioral impairments, and/or endocrine dysfunction. Conventional neuroimaging highlighted a spectrum of abnormalities including hypoplasia of oculomotor nerves, and dysgenesis of the corpus callosum, anterior commissure, and corticospinal tracts. A knock-in mouse model revealed axon guidance defects without evidence of cortical cell migration abnormalities. Several of the disease-associated TUBB3 substitutions reside at putative kinesin interaction sites on -tubulin, and we found that microtubules from knock-in mice had decreased interactions with the CFEOM1 gene product, Kif21a. Moreover, modeling the entire allelic series of mutations in yeast confirmed that a subset disrupts microtubule-kinesin interactions. We now propose both translational and basic Aims to continue our studies of CFEOM and the TUBB3 syndromes, as well as to explore the relationship of these disorders of axon guidance to the cytoskeleton and kinesin transport. We will further determine and define the TUBB3 phenotypes, including through the use of diffusion spectrum imaging to visualize white matter tracts in living participants and in ex vivo mouse models. Using an allelic series of Tubb3 mice, we will define the role of normal and mutant Tubb3 in neurodevelopment and axon guidance in vivo and in vitro. We hypothesize that perturbations to kinesin protein trafficking, including but not limited to KIF21A, account for at least a subset of the TUBB3 disease phenotypes. Thus, we will use quantitative mass spectrometry to identify and characterize altered kinesin-microtubule interactions, thereby also potentially defining novel roles for kinesin proteins in axon guidance. Finally, given the success of our genetic approach to identify disorders of axon guidance, we will continue our ongoing ascertainment and genetic analysis of CFEOM in order to identify additional disease genes. Following the completion of these Aims, we will have phenotypically defined one of the few known genetic disorders of axon guidance, thus aiding our understanding of human neurological diseases. Moreover, we will relate this disorder mechanistically to perturbations of the cytoskeleton and motor protein trafficking, defining the role of TUBB3 in axon guidance and highlighting the requirement of a specific tubulin isotype for proper neuronal function.